1. Field of the Invention
This invention relates to improvements in electronic sensors and switches, and, more particularly, to improvements in switch and sensor designs employing pultruded contact members. The invention also relates to a sensor using pultruded contact members to determine the presence of an object such as a sheet of paper in a detection zone.
2. Description of the Prior Art
Many switch and sensor designs and configurations have been proposed in the past. Generally most switches employ a movable member that carries a conductor in a manner such that it can be selectively brought into electrical contact with another conductor. Generally, most sensors employ two members having conductors in electrical contact, wherein the electrical contact is interrupted by the object to be sensed as the object passes between the members. The conductors are generally metal, but other conducive materials have been used as well.
As will become apparent, the invention has wide applications; however, a preferred embodiment of the invention is particularly suitable for applications in electrostatographic reproducing machines. In a typical electrostatographic reproducing machine, a photoconductive insulating surface, often in the form of a moving belt, is uniformly charged and exposed to a light image from an original document. The light image causes the exposed or background areas to become discharged, and creates an electrostatic latent image on the surface corresponding to the image contained within the original document. Alternatively, a light beam such as a laser beam may be modulated and used to selectively discharge portions of the photoconductive surface to record the desired information thereon. The electrostatic latent image is made visible by developing the image with a developer powder, referred to in the art as toner, which may be subsequently transferred to a support surface such as paper to which it may be permanently affixed by the application of heat and/or pressure.
In order to insure that the image is properly registered on the paper, switches or sensors are provided to detect the presence of the paper as it is moved through the various processing stages of the machine. However, because of the relatively hostile environment within such electrostatographic reproducing machines due to the presence of toner and other possible contaminants, the detecting switches and sensors need to be made sturdy and reliable, adding to the complexity and expense of the machine.
In addition, as the paper is moved though the machine, it may pick up stray or undesirable static charges that may interfere with the quality of the latent image, and ultimately the final image that is produced on it. Consequently, steps need to be taken to control such stray charges that may exist on the paper.
In many applications, it is desired to reduce the size of the switches and sensors that are used, but mechanical considerations limit the size of reduction that can be achieved. Moreover, many applications, such as, for example, in electrostatic reprographic machines, present hostile environments in which toner and other particulates exist, requiring the switch and sensor contacts be sufficiently large to insure reliable electrical connection when the contacts are brought together. In attempts to accomplish reliable switch and sensor structures, special expensive materials and designs have been proposed, yet room exists for further improvement toward achieving switches and sensors of miniature and microminiature design.
Another problem that exists, particularly with metal contacts, is that oxide often forms on the metal of the contacts, further reducing the reliability of the switch, particularly in hostile environments.
U.S. Pat. No. 4,641,949 describes the application of carbon fibers combined to form the basis of a non-metallic contact which in turn is the basis of a paper sensor.